Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy, which typically uses a linear accelerator (LINAC) to generate x-rays. In one type of external beam radiation therapy, an external radiation source directs a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to and from the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiosurgery” refers to a procedure in which radiation is applied to a target region at levels that are sufficient to necrotize a pathology. Radiosurgery is typically characterized by relatively high radiation doses per treatment (e.g., 1000-2000 centiGray), extended treatment times (e.g., 45-60 minutes per treatment) and hypo-fractionation (e.g., one to three days of treatment). The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centiGray), shorter treatment times (e.g., 10 to 30 minutes per treatment) and hyper-fractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to mean radiosurgery and/or radiotherapy unless otherwise noted.
Image-guided radiation treatment (IGRT) systems include gantry-based systems and robotic-based systems. In gantry-based systems, the radiation source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. The radiation source may be rigidly attached to the gantry or attached by a gimbaled mechanism. Each time a radiation beam is delivered during treatment, the axis of the beam passes through the isocenter. Treatment locations are therefore limited by the rotation range of the radiation source, the angular range of the gimbaled mechanism and the degrees of freedom of a patient positioning system. In robotic-based systems, such as the CyberKnife® Robotic Radiosurgery System manufactured by Accuray Incorporated of California, the radiation source is not constrained to a single plane of rotation and has five or more degrees of freedom.
In conventional image-guided radiation treatment systems, patient tracking during treatment is accomplished by comparing two-dimensional (2D) in-treatment x-ray images of the patient to 2D digitally reconstructed radiographs (DRRs) derived from three dimensional (3D) pre-treatment diagnostic imaging data of the patient. The pre-treatment imaging data may be computed tomography (CT) data, cone-beam CT, magnetic resonance imaging (MRI) data, positron emission tomography (PET) data or 3D rotational angiography (3DRA), for example. Typically, the in-treatment x-ray imaging system is stereoscopic, producing images of the patient from two or more different points of view (e.g., orthogonal projections).
A DRR is a synthetic x-ray image generated by casting (mathematically projecting) rays through the 3D imaging data, simulating the geometry of the in-treatment x-ray imaging system. The resulting DRR then has the same scale and point of view as the in-treatment x-ray imaging system, and can be compared with the in-treatment x-ray images to determine the position and orientation of the patient (and the radiation target within the patient). Different patient poses are simulated by performing 3D transformations (rotations and translations) on the 3D imaging data before each DRR is generated.
Each comparison of an in-treatment x-ray image with a DRR produces a similarity measure or equivalently, a difference measure, which can be used to search for a 3D transformation that produces a DRR with a higher similarity measure to the in-treatment x-ray image. When the similarity measure is sufficiently maximized (or equivalently, a difference measure is minimized), the corresponding 3D transformation can be used to align the patient in the radiation treatment system so that the actual treatment conforms to the treatment plan.
In conventional image-guided systems, the center of the imaging system (imaging center) and the center of the radiation treatment system (treatment center) are approximately collocated, which allows the imaging, patient alignment and treatment operations to be closely coupled during the treatment session.
However, co-location of the two systems may have disadvantages in some cases. In one case, one or more components of the imaging system (e.g., an imaging x-ray source or detector) may physically block the movement of the radiation treatment source to a desired/required treatment location. In another case, the radiation treatment source may block one of the x-ray imaging beam paths and interfere with stereoscopic imaging. In yet another case, patient positioning within the treatment delivery system may be restricted by the imaging system.
One type of gantry-based IGRT system, known as a portal imaging system, uses one or more LINACs at one or more positions to perform radiation treatment and imaging. FIG. 1 illustrates a conventional gantry-based IGRT system 100 with portal imaging. In FIG. 1, gantry 101 includes LINAC(s) 102 and a portal imaging detector 103 that rotate around an isocenter 104 where a patient on a treatment couch would be located for treatment. In one version, the system includes one LINAC that operates at a high energy level for radiation treatment and at a lower energy level for imaging. In another version, the system may include one high energy LINAC for radiation treatment and a second, lower energy LINAC for imaging. For radiation treatment, the radiation beam is collimated and/or shaped to concentrate radiation on a targeted pathology. For imaging, the beam is un-collimated to generate a wider field of view.
Portal imaging has some significant disadvantages. The images have low contrast because the high-energy radiation treatment beam is used for imaging and the differential absorption of different tissue types is low. The images also have more “noise” due to Compton scattering and secondary electron emissions associated with the high-energy beam. Another disadvantage is that because there is only one x-ray source and one x-ray detector for imaging, the gantry must be rotated between two positions in order to generate a pair of stereoscopic images. These disadvantages render portal imaging a poor candidate as the primary imaging system in an image-guided radiation treatment system.